Porous electrode substrate, method for producing the same, membrane electrode assembly, and polymer electrolyte fuel cell

ABSTRACT

Provided is a porous electrode substrate having high mechanical strength, good handling properties, high thickness precision, little undulation, and adequate gas permeability and conductivity. Also provided is a method for producing a porous electrode substrate at low costs. A porous electrode substrate is produced by joining short carbon fibers (A) via mesh-like of carbon fibers (B) having an average diameter of 4 μm or smaller. Further provided are a membrane-electrode assembly and a polymer electrolyte fuel cell that use this porous electrode membrane. A porous electrode substrate is obtained by subjecting a precursor sheet, in which short carbon fibers (A) and short carbon fiber precursors (b) having an average diameter of 5 μm or smaller have been dispersed, to carbonization treatment after optional hot press forming and optional oxidization treatment.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a 35 U.S.C. §371 national stage patentapplication of International patent application PCT/JP10/051,380, filedon Feb. 2, 2010, and claims priority to Japanese Patent Application no.JP2009-023890, filed on Feb. 4, 2009.

TECHNICAL FIELD

The present invention relates to a porous electrode substrate used in apolymer electrolyte fuel cell using a gas fuel and a liquid fuel, amembrane electrode assembly using the same, and a polymer electrolytefuel cell.

BACKGROUND ART

Polymer electrolyte fuel cells are characterized by using a protonicconductive polymer electrolyte membrane, and are devices that provide anelectromotive force by electrochemically reacting a fuel gas such ashydrogen with an oxidizing gas such as oxygen. The polymer electrolytefuel cells can be utilized as private power generators and powergenerators for mobile bodies such as automobiles.

Such a polymer electrolyte fuel cell has a polymer electrolyte membranewhich selectively conducts hydrogen ions (protons). The fuel cell hastwo sets of gas diffusion electrodes and has a structure as describedbelow. The gas diffusion electrode has a catalyst layer which contains,as a main component, a carbon powder which supports a noble metal-basedcatalyst, and has a gas diffusion electrode substrate. Each of the gasdiffusion electrodes is joined to the surface of a polymer electrolytemembrane with the catalyst layer facing inward.

An assembly composed of such a polymer electrolyte membrane and two setsof such gas diffusion electrodes is referred to as a membrane electrodeassembly (MEA). On both outer sides of the MEA, separators are installedin which gas flow paths are formed in order to feed a fuel gas and anoxidizing gas and to discharge produced gases and excessive gases.

A gas diffusion electrode substrate needs mechanical strength becausethe gas diffusion electrode substrate is fastened by a load of severalmegapascals by a separator in order to reduce the electric contactresistance and suppress the leakage of a fuel gas or an oxidizing gasfed from the separator to the outside a fuel cell.

Since a gas diffusion electrode substrate needs to mainly have thefollowing three functions, the gas diffusion electrode substrate isusually a porous electrode substrate having a porous structure. A firstfunction required for the gas diffusion electrode substrate is thefunction of uniformly feeding a fuel gas or an oxidizing gas, from a gasflow path formed in a separator which is arranged outer side of the gasdiffusion electrode substrate, to a noble metal-based catalyst in acatalyst layer. A second function is a function of discharging waterproduced by a reaction in the catalyst layer. A third function is afunction of conducting electrons necessary for a reaction in thecatalyst layer or electrons produced by a reaction in the catalyst layerto the separator. What is considered to be an effective way to realizethese functions is to employ a gas diffusion electrode substrate thatgenerally uses a carbonaceous material.

Conventionally, in order to increase mechanical strength of thesubstrate, short carbon fibers were formed to a paper and bound oneanother by using organic polymers, and then this paper is firing at ahigh temperature to carbonize the organic polymers and to produce aporous electrode substrate which is composed of carbon/carbon compositesin paper shape. However, the production process is complicated and aproblem thereof is high cost. Although, in order to reduce the cost, aporous electrode substrate is proposed which is obtained by forming apaper from oxidized short fibers, and thereafter firing the paper at ahigh temperature to carbonize the oxidized short fibers, since theoxidized short fibers shrink during firing, problems of the electrodesubstrate are the dimensional stability thereof and a large undulation(the state of the sheet cross-section being waved or the state of thatbeing warped).

Patent Literature 1 discloses a porous carbon electrode substrate for afuel cell having features that include a thickness of 0.05 to 0.5 mm anda bulk density of 0.3 to 0.8 g/cm³, and a bending strength of 10 MPa orhigher and a deflection in bending of 1.5 mm or more in a 3-pointbending test under the conditions of a strain rate of 10 mm/min, adistance between fulcrums of 2 cm and a test piece width of 1 cm.However, although the porous electrode substrate exhibits highmechanical strength, small undulation, sufficient gas permeability andsufficient electroconductivity, the problem thereof is high productioncost.

Patent Literature 2 discloses a carbon fiber sheet having a thickness of0.15 to 1.0 mm, a bulk density of 0.15 to 0.45 g/cm³, a carbon fibercontent of 95% by mass or more, a compression deformation ratio of 10 to35%, an electric resistivity of 6 mΩ or lower, and a degree of drape of5 to 70 g. Although this method for producing the carbon fiber sheet canbe at a low cost, since shrinkage during firing is large, problems thatoccur in the resulting porous electrode substrate include a largeunevenness in the thickness and large undulation.

Patent Literature 3 discloses a porous electrode substrate which isobtained by carbonizing a sheet composed of carbon fibers and acrylicpulp fibers. Although the porous electrode substrate can be produced ata low cost, since there is little entanglement between the carbon fibersand the acrylic pulp fibers during the process of forming the sheet,handling the porous electrode substrate is difficult. Comparing theacrylic pulp fibers with common fibrous materials, since the polymerexhibits almost no molecular orientation, the carbonization ratio duringcarbonization is low; thus in order to raise the handleability, much ofthe acrylic pulp fiber needs to be added.

CITATION LIST Patent Literature

-   Patent Literature 1: WO 2002/042534-   Patent Literature 2: WO 2001/056103-   Patent Literature 3: JP2007-273466A

SUMMARY OF INVENTION Technical Problem

The present invention overcomes the above-mentioned problems, andprovides a porous electrode substrate which exhibits little breakageduring pressurization, good handleability, high thickness precision andsmall undulation, and which has a sufficient gas permeability and asufficient electroconductivity, and a method for producing a porouselectrode substrate at a low production cost.

Solution to Problem

The present inventors have found that the above-mentioned problems canbe solved by the following inventions [1] to [6].

[1] A porous electrode substrate obtained by joining short carbon fibers(A) via mesh-like carbon fibers (B) having an average diameter of 4 μmor smaller.

[2] A method for producing a porous electrode substrate, including astep (1) of producing a precursor sheet containing short carbon fibers(A) and short carbon fiber precursors (b) having an average diameter of5 μm or smaller dispersed therein, and a step (2) of subjecting theprecursor sheet to carbonization treatment at a temperature of 1,000° C.or higher.[3] The method for producing a porous electrode substrate according tothe above [2], wherein the method includes a step (3) of subjecting theprecursor sheet to hot press forming at a temperature of lower than 200°C. between the step (1) and the step (2).[4] The method for producing a porous electrode substrate according tothe above [3], wherein the method includes a step (4) of subjecting theprecursor sheet, which has been subjected to hot press forming, tooxidization treatment at a temperature between 200° C. or higher andlower than 300° C. between the step (3) and the step (2).[5] A membrane electrode assembly using a porous electrode substrateaccording to the above [1].[6] A polymer electrolyte fuel cell using a membrane electrode assemblyaccording to the above [5].

Advantageous Effects of Invention

The porous electrode substrate according to the present inventionexhibits little breakage during compression, good handleability andsmall undulation, and has a sufficient gas permeability and a sufficientelectroconductivity. The method for producing a porous electrodesubstrate according to the present invention is at low cost because itdoes not need a resin-binding step.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a scanning electron microscopic photograph of a surface of theporous electrode substrate according to the present invention.

DESCRIPTION OF EMBODIMENTS

The porous electrode substrate according to the present invention cantake the shape of a sheet, a spiral or the like. The basis weight of theporous electrode substrate in the shape of a sheet is preferably about15 to 100 g/m². The thickness is preferably about 50 to 300 μm. The gaspermeability is preferably about 500 ml/hr/cm²/mmAq or higher. Thethrough-plane electric resistance (electric resistance in the thicknessdirection) is preferably 50 mΩ·cm² or lower. Here, the measurementmethods of the gas permeability and the through-plane electricresistance will be described later.

Short carbon fibers (A) constituting a porous electrode substrate aredispersed planarly. “Being dispersed planarly” means that short carbonfibers (A) are present parallel or nearly parallel to the surface of asheet-shaped electrode substrate. Since the short carbon fibers (A) aredispersed in such a manner, a short circuit due to the short carbonfibers (A) and the breakage of the short carbon fibers (A) can beprevented. The short carbon fibers (A) in a plane may be substantiallyrandomly orientated, or may be orientated highly in a specificdirection. The short carbon fibers (A) are present with a linear shapeheld in a porous electrode substrate. In a porous electrode substrate,the short carbon fibers (A) are not directly bonded, but joined viamesh-like carbon fibers (B)

The short carbon fibers (A) constituting a porous electrode substrateinclude carbon fibers obtained by cutting carbon fibers, such as apolyacrylonitrile-based carbon fiber (hereinafter, referred to as“PAN-based carbon fiber” in some cases), a pitch-based carbon fiber anda rayon-based carbon fiber, into a suitable length. The fiber length ispreferably about 2 to 12 mm from the viewpoint of dispersibility. APAN-based carbon fiber is preferable from the viewpoint of mechanicalstrength of a porous electrode substrate. The diameter of a short carbonfiber (A) is preferably 3 to 9 μm from the viewpoint of production costand dispersibility of the short carbon fiber. The diameter is morepreferably 4 μm or larger and 8 μm or smaller from the viewpoint oflessening the undulation of a porous electrode substrate.

The mesh-like carbon fibers (B) having an average diameter of 4 μm orlower are ones in which a mesh-like structure was formed by the fusionof short carbon fiber precursors (b) (described later, hereinafter,referred to as fibers (b) in some cases) having an average diameter of 5μm or lower, at portions of fibers (b) that came into contact with eachother and that came into contact with short carbon fibers (A) in aprecursor sheet (described later), and then the fibers (b) shrank. Theshape of the mesh-like carbon fibers (B) which connect one joiningportion of the mesh of the mesh-like carbon fibers (B) with anotherjoining portion, adjacent to the first joining portion, of the mesh ofthe mesh-like carbon fibers (B) or with still another joining portion,adjacent to the first joining portion at the short carbon fibers (A), isnearly linear. The mesh-like carbon fibers (B) are present in a bentstate or a curved state at joining portions with the short carbon fibers(A). If a porous electrode substrate is taken as 100% by mass, themesh-like carbon fibers (B) are more preferably 25% by mass or more and60% by mass or less in order to maintain the mechanical strength of theporous electrode substrate at a sufficient strength.

The porous electrode substrate according to the present invention can beproduced, for example, by the above-mentioned methods.

That is, a first production method is a method sequentially carrying outstep (1) of producing a precursor sheet containing short carbon fibers(A) and short carbon fiber precursors (b) having an average diameter of5 μm or smaller dispersed therein, and step (2) of subjecting theprecursor sheet to carbonization treatment at a temperature of 1,000° C.or higher. A second production method is a method for sequentiallycarrying out step (1), step (3) of subjecting the precursor sheet to hotpress forming at a temperature of lower than 200° C., and step (2). Athird production method is a method for sequentially carrying out step(1), step (3), step (4) of subjecting the precursor sheet which has beensubjected to the hot press forming to oxidization treatment at atemperature between 200° C. or higher and lower than 300° C., and step(2).

The fiber (b) is one obtained by cutting a long carbon fiber precursorinto a suitable length. The fiber length of the fiber (b) is preferablyabout 2 to 20 mm from the viewpoint of dispersibility. Thecross-sectional shape of fiber (b) is not especially limited, but ispreferably of a high roundness from the viewpoint of mechanical strengthafter carbonization and production cost. The diameter of the fiber (b)is preferably 5 μm or lower in order to suppress breakage due toshrinkage during carbonization. With the diameter of 5 μm or lower, manyof the joining points with the short carbon fibers (A) aftercarbonization can be secured, which is preferable.

Such a fiber (b) includes a polyacrylonitrile-based short carbon fiberprecursor (hereinafter, referred to as “PAN-based short carbon fiberprecursor” in some cases), a cellulose-based short carbon fiberprecursor, and a phenol-based short carbon fiber precursor. ThePAN-based short carbon fiber precursor is preferable taking intoconsideration that it can be joined with the short carbon fibers (A) ina range from a low temperature to a high temperature, and the remainingmass after carbonization is large. The proportion of the fibers (b)remaining as mesh-like carbon fibers (B) finally obtained variesdepending on the kind of the fibers (b), the mixing ratio with the shortcarbon fibers (A), and the presence/absence of oxidization treatment at200° C. or higher and 300° C. or less. The amount of the fibers (b) thatare used based on 100 parts by mass of the short carbon fibers (A) ispreferably about 50 to 300 parts by mass.

Applicable production methods of a precursor sheet are paper makingmethods including a wet method in which short carbon fibers (A) andfibers (b) are dispersed in a liquid medium, and subjected to papermaking, and a dry method in which short carbon fibers (A) and fibers (b)are dispersed in air, and made to fall and accumulate, but the wetmethod is preferable. It is preferable that a proper amount of fibers(b) be used in order to facilitate the dispersion of short carbon fibers(A) as single fibers and also to prevent the dispersed single fibersfrom reconverging, and that as required, an organic polymer compound beused as a binder and then the mixture of these materials be subjected toa wet paper making.

A method for mixing short carbon fibers (A), fibers (b) and organicpolymer compounds includes a method of stirring and dispersing thesematerials in water, and a method of directly mixing these materials, butis preferably a method of stirring and dispersing these materials inwater in order to disperse these materials homogeneously. By mixingshort carbon fibers (A) and fibers (b), and further optionally organicpolymer compounds as required, and forming a paper from them to producea precursor sheet, the strength of the precursor sheet is improved, andexfoliation of the short carbon fibers (A) from the precursor sheet anda change in the orientation of the short carbon fibers (A) duringproduction can be prevented.

A precursor sheet can be produced by either a continuous method or abatch method, but is preferably produced by a continuous method from theviewpoint of productivity and mechanical strength of the precursorsheet.

The basis weight of a precursor sheet is preferably about 10 to 200g/m². The thickness thereof is preferably about 20 to 200 μm.

The organic polymer compound functions as a binder (pasting agent) totie each component in a precursor sheet containing short carbon fibers(A) and fibers (b). Organic polymer compounds that can be used are apolyvinyl alcohol (PVA), a polyvinyl acetate and the like. Polyvinylalcohol, in particular, is preferable because it has excellent bindingpower characteristics during the paper making process and because thereis little fall-off of the short carbon fibers. In the present invention,an organic polymer compound may be used by forming it into a fibershape.

A precursor sheet may be subjected to carbonization treatment, as is, ormay be subjected to carbonization treatment after being subjected to hotpress forming. After hot press forming and followed by oxidizationtreatment, a precursor sheet may also be subjected to carbonizationtreatment. Short carbon fibers (A) are joined by melting fibers (b) andmesh-like carbon fibers (B) having an average diameter of 4 μm or lowerare produced by carbonizing the fibers (b). Thereby, a porous electrodesubstrate can be developed so that it has mechanical strength andelectroconductivity. Carbonization treatment is preferably carried outin an inert gas in order to increase the electroconductivity of a porouselectrode substrate. Carbonization treatment is usually carried out at atemperature of 1,000° C. or higher. Subjecting a precursor sheet tocarbonization treatment in the temperature range of 1,000 to 3,000° C.is preferable, and a temperature range of 1,000 to 2,200° C. is morepreferable. If the carbonization treatment temperature is too low, theelectroconductivity of a porous electrode substrate becomesinsufficient. Before carbonization treatment, a pre-treatment of firingin an inert atmosphere at about 300 to 800° C. can be carried out. Theduration of carbonization treatment is, for example, about 10 minutes to1 hour.

In the case where a precursor sheet continuously produced is subjectedto carbonization treatment, the carbonization treatment is preferablycarried out continuously over the entire length of the precursor sheetfrom the viewpoint of reducing production costs. If a porous electrodesubstrate has a long length, since productivity of the porous electrodesubstrate is increased, and a MEA production thereafter can be carriedout continuously, the production cost of a fuel cell can be greatlyreduced. A porous electrode substrate is preferably rolled upcontinuously from the viewpoint of productivity and reduction of theproduction cost of the porous electrode substrate and a fuel cell.

A precursor sheet is preferably subjected to hot press forming at atemperature of lower than 200° C. before carbonization treatment fromthe viewpoint that short carbon fibers (A) and fibers (b) are joined bymelting fibers (b) and unevenness in the thickness of a porous electrodesubstrate is reduced. Any technology can be applied to the hot pressforming as long as the technology is capable of applying a uniform hotpress forming process to the precursor sheet. For example, thetechnology includes a method in which flat and smooth rigid plates areabutted on both surfaces of a precursor sheet, and thermally pressed,and a method using a continuous belt press apparatus.

Taking into consideration the production of a long porous electrodesubstrate, in the case where a precursor sheet that is continuouslyproduced is subjected to hot press forming, the method using acontinuous belt press apparatus is preferable. If a porous electrodesubstrate has a long length, since productivity of the porous electrodesubstrate is increased, and a MEA production thereafter can be carriedout continuously, reduction of the production costs of a fuel cell canbe achieved. Taking into consideration productivity and reduction of theproduction costs of the porous electrode substrate and the fuel cell,the long porous electrode substrate is preferably rolled upcontinuously. The press method in the continuous belt press apparatusinvolves a method in which pressure is applied as a line pressure on abelt with a roll press, and a method in which the belt is pressed byusing surface pressure with a liquid-pressure head press, but the latteris preferable from the viewpoint of being capable of providing a moreflat and smooth porous electrode substrate.

The heating temperature during hot press forming is preferably lowerthan 200° C., and more preferably 120 to 190° C., in order toeffectively make the surface of a precursor sheet flat and smooth. Theforming pressure is not especially limited, but in the case where thecontent ratio of fibers (b) in a precursor sheet is high, the surface ofthe precursor sheet can be made flat and smooth easily even if theforming pressure is low. At this time, if the pressing pressure is madehigher than necessary, problems that can occur include that short carbonfibers (A) break during hot press forming and the structure of a porouselectrode substrate is too dense. The forming pressure is preferablyabout 20 kPa to about 10 MPa. The duration of the hot press forming canbe made to last, for example, from 30 seconds to 10 minutes. When aprecursor sheet is subjected to hot press forming by being interposedbetween two sheets of rigid plates or with a continuous belt pressapparatus, it is preferable that a release agent be previously appliedand that mold-releasing paper be interposed between the precursor sheetand the rigid plate or the belt so that the fibers (b) or the like donot adhere to the rigid plate or the belt.

A precursor sheet is preferably subjected to oxidization treatment at atemperature between 200° C. or higher and lower than 300° C. after beingsubjected to hot press forming, from the viewpoint of satisfactorilyjoining short carbon fibers (A) and fibers (b) by melting fibers (b),and improving the carbonization ratio of fibers (b). Oxidizationtreatment is more preferably carried out at 240 to 270° C. Continuousoxidization treatment by pressurizing and direct heating by using aheating porous plate, or continuous oxidization treatment by anintermittent pressurizing and direct heating by using a heating roll orthe like is preferable from the viewpoint of low cost and of beingcapable of satisfactorily joining short carbon fibers (A) and fibers (b)by melting fibers (b). The duration of oxidization treatment can be madeto last, for example, from 1 minute to 2 hours. In the case where aprecursor sheet that is continuously produced is subjected tooxidization treatment, the oxidization treatment is preferably carriedout continuously over the entire length of the precursor sheet. Thereby,carbonization treatment can be carried out continuously, andproductivity of a porous electrode substrate, a MEA and a fuel cell canbe improved and production costs can be reduced.

The porous electrode substrate according to the present invention cansuitably be used for a membrane electrode assembly. A membrane electrodeassembly using the porous electrode substrate according to the presentinvention can suitably be used for a polymer electrolyte fuel cell.

EXAMPLES

Hereinafter, the present invention will be described more specificallywith reference to Examples. Physical properties and the like in Exampleswere measured by the following methods. “Parts” means “parts by mass.”

(1) Gas Permeability

According to JIS P-8117, the time taken for 200 mL of air to permeate aporous electrode substrate was measured using a Gurley Densometer, andthe gas permeability (ml/hr/cm²/mmAq) was calculated.

(2) Thickness

The thickness of a porous electrode substrate was measured using athickness measuring device, Dial Thickness Gauge (trade name: 7321, madeby Mitsutoyo Co., Ltd.). The size of the gauge head was 10 mm indiameter, and the measurement pressure was set at 1.5 kPa.

(3) Through-Plane Electric Resistance

The electric resistance (through-plane electric resistance) in thethickness direction of a porous electrode substrate was determined fromthe following expression by interposing the porous electrode substratebetween gold-plated copper plates, pressurizing the copper plates fromabove and below at 1 MPa, and measuring the resistance value whencurrent was allowed to flow at a current density of 10 mA/cm².A through-plane electric resistance (mΩ·cm²)=a measured resistance value(mΩ)×a sample area (cm²)(4) Average Diameter of Mesh-Like Carbon Fibers (B)

The diameters of arbitrary 50 points of mesh-like carbon fibers (B) weremeasured from scanning electron microscopic photographs of the surfaceof a porous electrode substrate, and the average diameter was calculatedfrom the measurement.

(5) Content of Mesh-Like Carbon Fibers (B)

The content of mesh-like carbon fibers (B) was calculated by thefollowing expression from the basis weight of a porous electrodesubstrate that was obtained and from the basis weight of short carbonfibers (A) that were used.A content of mesh-like carbon fibers (B)(%)=[a basis weight of a porouselectrode substrate (g/m²)−a basis weight of short carbon fibers(A)(g/m²)]/the basis weight of the porous electrode substrate (g/m²)×100(6) Undulation of a Porous Electrode Substrate

The undulation of a porous electrode substrate was calculated from thedifference between the maximum value and the minimum value of heights ofthe porous electrode substrate when the porous electrode substratehaving a length of 250 mm and having a width of 250 mm was left at reston a flat plate.

(7) Handleability of a Porous Electrode Substrate

When a porous electrode substrate was handled, if the shape thereofcould be maintained (when a porous electrode substrate was held up,there occurred no smashing, no fracturing, no breaking, no tearing andotherwise), the handleability was considered good. By contrast, ifsmashing, fracturing, breaking, tearing and the like occurred, thehandleability was considered bad.

Example 1

As short carbon fibers (A), PAN-based carbon fibers having an averagefiber diameter of 7 μm and an average fiber length of 3 mm wereprepared. As short carbon fiber precursors (b) having an averagediameter of 5 μm or lower, acrylic short fibers having an average fiberdiameter of 4 μm and an average fiber length of 3 mm (trade name: D122,made by Mitsubishi Rayon Co., Ltd.) were prepared. Further as an organicpolymer compound, polyvinyl alcohol (PVA) short fibers having an averagefiber length of 3 mm (trade name: VBP105-1, made by Kuraray Co., Ltd.)were prepared.

At first, 100 parts of the short carbon fibers (A) were dispersed inwater. When the short carbon fibers (A) were sufficiently andhomogeneously dispersed, 83 parts of the acrylic short fibers and 53parts of the PVA short fibers were added into the suspension and werehomogeneously dispersed. Then, the dispersion liquid was manually spreadin a planar shape to form a paper (having a length of 250 mm and havinga width of 250 mm) by using a standard square sheet machine (trade name:No. 2555, made by Kumagai Riki Kogyo Co., Ltd.) according to JIS P-8209,and dried to obtain a precursor sheet of 36 g/m² in basis weight. Thedispersion condition of the short carbon fibers (A) and the acrylicshort fibers was good.

Then, two sheets of the precursor sheet were overlapped; both surfacesthereof were interposed between papers which are coated with asilicone-based die-releasing agent, and the laminate was thereaftersubjected to hot press forming for 3 minutes under the conditions of180° C. and 3 MPa by using a batch press apparatus. Then, both surfacesof the precursor sheet were interposed between stainless steel punchingplates which are coated with a silicone-based release agent, andthereafter subjected to oxidization treatment for 1 minute under theconditions of 280° C. and 0.5 MPa by using a batch press apparatus.Thereafter, the precursor sheet which has been subjected to oxidizationtreatment was subjected to carbonization treatment for 1 hour under thecondition of 2,000° C. in a nitrogen gas atmosphere in a batchcarbonization furnace to obtain a porous electrode substrate.

The porous electrode substrate exhibited almost no in-plane shrinkageduring carbonization treatment, exhibited good handleability and smallundulation of 2 mm or less, and had good air permeability, thickness andthrough-plane electric resistance. The average diameter of the mesh-likecarbon fibers (B) was 3 μm; and the content thereof was 32% by mass. Ascanning electron microscopic photograph of the surface of the porouselectrode substrate is shown in FIG. 1. It can be confirmed that thedispersed short carbon fibers (A) were joined via mesh-like carbonfibers (B). The evaluation results are shown in Table 1.

Examples 2 and 3, and 11 to 13

Porous electrode substrates were obtained as in Example 1, except forsetting the amounts of the acrylic short fibers and the PVA short fibersthat were used and the basis weights of the precursor sheet at valuesshown in Table 1. The porous electrode substrates exhibited almost noin-plane shrinkage during carbonization treatment, exhibited goodhandleability and small undulation of 2 mm or less, and had good airpermeability, thickness and through-plane electric resistance. Theplanarly dispersed short carbon fibers (A) were joined via mesh-likecarbon fibers (B). The evaluation results are shown in Table 1.

Example 14

A porous electrode substrate was obtained as in Example 1, except forsetting the amounts of the acrylic short fibers and the PVA short fibersthat were used and the basis weight of the precursor sheet at valuesshown in Table 1, and subjecting one sheet of the precursor sheet to hotpressure forming. The porous electrode substrate exhibited almost noin-plane shrinkage during carbonization treatment, exhibited goodhandleability and small undulation of 2 mm or less, and had good airpermeability, thickness and through-plane electric resistance. Theplanarly dispersed short carbon fibers (A) were joined via mesh-likecarbon fibers (B). The evaluation results are shown in Table 1.

Example 6

A porous electrode substrate was obtained as in Example 1, except forusing acrylic short fibers having an average fiber diameter of 3 μm andan average fiber length of 3 mm (trade name: D125, made by MitsubishiRayon Co., Ltd.) as short carbon fiber precursors (b) having an averagediameter of 5 μm or lower. The porous electrode substrate exhibitedalmost no in-plane shrinkage during carbonization treatment, exhibitedgood handleability and small undulation of 2 mm or less, and had goodair permeability, thickness and through-plane electric resistance. Theplanarly dispersed short carbon fibers (A) were joined via mesh-likecarbon fibers (B). The evaluation results are shown in Table 1.

Examples 7, and 15 and 16

Porous electrode substrates were obtained as in Example 6, except forsetting the amounts of the acrylic short fibers and the PVA short fibersthat were used and the basis weights of the precursor sheet at valuesshown in Table 1. The porous electrode substrates exhibited almost noin-plane shrinkage during carbonization treatment, exhibited goodhandleability and small undulation of 2 mm or less, and had good airpermeability, thickness and through-plane electric resistance. Theplanarly dispersed short carbon fibers (A) were joined via mesh-likecarbon fibers (B). The evaluation results are shown in Table 1.

Example 8

A porous electrode substrate was obtained as in Example 1, except thatno oxidization treatment was carried out. The structure and performanceof the porous electrode substrate were good as in Example 1. Theevaluation results are shown in Table 1.

Example 9

A porous electrode substrate was obtained as in Example 1, except thatno hot press treatment and no oxidization treatment were carried out.The structure and performance of the porous electrode substrate weregood as in Example 1. The evaluation results are shown in Table 1.

Example 10 (1) Production of a Membrane Electrode Assembly (MEA)

Two sets of porous electrode substrates obtained in Example 1 wereprepared as porous electrode substrates for a cathode and an anode. Alaminate was prepared in which a catalyst layer (catalyst layer area: 25cm², amounts of Pt deposited: 0.3 mg/cm²), which contained acatalyst-supported carbon (catalyst: Pt, amounts of catalyst supported:50% by mass), was formed on both surfaces of a perfluorosulfonicacid-based polymer electrolyte membrane (membrane thickness: 30 μm). Thelaminate was interposed between the porous electrode substrates for acathode and an anode, and these were joined to obtain a MEA.

(2) Evaluation of Properties of a Fuel Cell of the MEA

The MEA was interposed between two sheets of carbon separators havingbellows-like gas flow paths to form a polymer electrolyte fuel cell(unit cell). The current density-voltage properties were measured toevaluate properties of the fuel cell. A hydrogen gas was used as a fuelgas; and air was used as an oxidizing gas. The temperature of the unitcell was set at 80° C.; the utility factor of the fuel gas, 60%; and,the utility factor of the oxidizing gas, 40%. The humidification of thefuel gas and the oxidizing gas was carried out by passing the gasesthrough bubblers of 80° C., respectively. As a result, the cell voltageof the fuel cell at a current density of 0.8 A/cm² was 0.639 V; theinternal resistance of the cell was 3.3 mΩ, which exhibited goodproperties.

Comparative Example 1

A porous electrode substrate was obtained as in Example 1, except fornot using the acrylic short fibers but 133 parts of the PVA short fiberswere used, and the basis weight of the precursor sheet was set at 35g/m². In the porous electrode substrate, since the PVA was almost notcarbonized, the short carbon fibers (A) were not joined and thestructure of the sheet shape could not be maintained.

Comparative Example 2

A porous electrode substrate was obtained as in Example 2, except fornot using the short carbon fibers (A) but only 100 parts of the acrylicshort fibers and 16 parts of the PVA short fibers were used, and thebasis weight of the precursor sheet was set at 58 g/m². In the porouselectrode substrate, the structure of the sheet shape could not bemaintained due to the shrinkage thereof when the acrylic short fiberswere carbonized.

Comparative Example 3

A porous electrode substrate was obtained as in Example 1, except forusing acrylic short fibers having an average fiber diameter of 10 μm andan average fiber length of 10 mm as the short carbon fiber precursors(b). In the porous electrode substrate, it was observed that the acrylicshort fibers were broken at binding portions with the short carbonfibers due to the shrinkage during the carbonization. Additionally, themesh-like structure was not formed. The through-plane electricresistance exhibited larger resistance than that of the porous electrodesubstrate in Example 1. The evaluation results are shown in Table 1. Inthe porous electrode substrate, the fiber diameter of the carbon fibersthat originated from the acrylic short fibers was 7 μm, and the contentthereof was 26% by mass.

Comparative Examples 4 and 5

Porous electrode substrates were obtained as in Example 1, except forsetting the amounts of the acrylic short fibers and the PVA short fibersthat were used and the basis weights of the precursor sheet at valuesshown in Table 1. The porous electrode substrates exhibited almost noin-plane shrinkage during the carbonization treatment, goodhandleability and small undulation of 2 mm or less, and had good airpermeability, thickness and through-plane electric resistance. Althoughthe planarly dispersed short carbon fibers (A) were joined via mesh-likecarbon fibers (B), more breakage due to compression exerted during MEAproduction was observed than in the porous electrode substrate inExample 1. The evaluation results are shown in table 1.

Comparative Examples 6 and 7

Porous electrode substrates were obtained as in Example 6, except forsetting the amounts of the acrylic short fibers and the PVA short fibersthat were used and the basis weights of the precursor sheet at valuesshown in Table 1. The porous electrode substrates exhibited almost noin-plane shrinkage during the carbonization treatment, exhibited goodhandleability and small undulation of 2 mm or less, and had good airpermeability, thickness and through-plane electric resistance. Althoughthe planarly dispersed short carbon fibers (A) were joined via mesh-likecarbon fibers (B), more breakage due to compression exerted during MEAproduction was observed than in the porous electrode substrate inExample 6. The evaluation results are shown in table 1.

TABLE 1 short carbon short basis mean contents short mean diameter fiberPVA weight diameter of of thickness of carbon of short carbon precursorsfiber of a mesh-like mesh-like a porous through- fibers (A) fiberprecursors (b) (parts precursor cabon fibers cabon electrode gas planeelectric (parts by (b) (parts by by sheet (B) fibers (B) substratepermeability resistance mass) (μm) mass) mass) (g/m²) (μm) (%) (μm)(ml/hr/cm²/mmAq) (mΩ · cm²) Example 1 100 4 83 53 36 3 32 216 2400 7.2Example 2 100 4 250 80 43 3 59 183 2200 7.2 Example 3 100 4 100 57 36 343 205 2300 7.2 Comparative 100 4 750 160 51 3 81 116 1400 3.6 Example 4Comparative 100 4 25 40 33 3 11 210 2900 7.6 Example 5 Example 6 100 383 53 36 2.5 28 205 2000 6.8 Example 7 100 3 250 80 43 2.5 56 157 18006.8 Example 8 100 4 83 53 36 3 27 212 2500 7.2 Example 9 100 4 83 53 363 27 245 3000 7.8 Comparative 100 — — 133 35 — — — — — Example 1Comparative — 4 100 16 58 — — — — — Example 2 Comparative 100 10  83 5336 — — 205 3600 8.8 Example 3 Example 11 100 4 250 80 34 3 59 144 28006.2 Example 12 100 4 250 80 54 3 58 215 1900 8.5 Example 13 100 4 80 2019 3 26  89 5500 5.8 Example 14 100 4 250 80 68 3 58 140 2900 6.1Example 15 100 3 250 80 34 2.5 57 121 2500 5.9 Example 16 100 3 250 8054 2.5 55 180 1400 7.8 Comparative 100 3 750 160 51 2.5 82 117 1200 3.6Example 6 Comparative 100 3 25 40 33 2.5 14 215 2600 7.7 Example 7

The invention claimed is:
 1. A method for producing a porous electrodesubstrate, comprising: producing a precursor sheet comprising shortcarbon fibers (A) and short carbon fiber precursors (b) dispersedtherein, wherein the short carbon fiber precursors (b) have an averagediameter of 5 μm or smaller and a length of from 2 to 20 mm, and theshort carbon fiber precursors (b) are obtained by cutting long carbonfiber precursors; and then subjecting the precursor sheet tocarbonization treatment at a temperature of 1,000° C. or higher toobtain a porous electrode substrate in which the short carbon fibers (A)are joined via mesh carbon fibers (B) having an average diameter of 4 μmor smaller, wherein the mesh carbon fibers (B) are obtained bycontacting the short carbon fiber precursors (b) with the short carbonfibers (A).
 2. The method according to claim 1, further comprisingsubjecting the precursor sheet to hot press forming at a temperature oflower than 200° C. between the producing and the subjecting tocarbonization treatment.
 3. The method according to claim 2, furthercomprising subjecting the precursor sheet, which has been subjected tothe hot press forming, to oxidization treatment at a temperature between200° C. to less than 300° C., between the hot press forming and thesubjecting to carbonization treatment.
 4. The method according to claim1, wherein the short carbon fiber precursors (b) have an averagediameter of from 3 μm to 5 μm.
 5. The method according to claim 1,wherein the short carbon fiber precursors (b) have an average diameterof from 4 μm to 5 μm.
 6. The method according to claim 1, wherein theshort carbon fiber precursors (b) are selected from apolyacrylonitrile-based short carbon fiber precursor, a cellulose-basedshort carbon fiber precursor, and a phenol-based short carbon fiberprecursor.
 7. The method according to claim 1, wherein an amount of theshort carbon fiber precursors (b), based on 100 parts by mass of theshort carbon fibers (A) is from 50 to 300 parts by mass.